"that the only map to trust, in terms of portages, is the official QP map."

well yeah, but that one has errors also - they all do

I have several notes on my Fisher maps pointing out wrong locations for portages - I suppose if I bought a whole new set, maybe some of those errors would have been corrected by now, but likely not

as for portages marked that don't exist - some may have been abandoned or re-routed to a new location.  I like to look at the Ontario Basic Mapping map for portages - just like the campsites on there, there are some old portages indicated that are hard to find if you can find them at all